1. Field of the Invention
The present invention is directed to technology and design of efficient ultrasound transducers for high frequencies. The invention has special advantages where the highest frequencies are above 10 MHz, but has also applications for transducers at lower frequencies.
2. Description of the Related Art
Medical ultrasound imaging at frequencies above ˜10 MHz, has a wide range of applications for studying microstructures in soft tissues, such as the composition of small tumors or a vessel wall. Ultrasound transducers for medical applications are currently based on ferro-electric, ceramic plates as the active material, that vibrates in thickness mode. When polarized, the materials show piezoelectric properties with efficient electromechanical coupling. However, the characteristic impedance of the ceramic material (Zx˜33 MRayl) is much higher than that of the tissue load material (ZL˜1.6 MRayl). In order to get adequate thickness vibration amplitude of the plate for efficient power coupling into the tissue load material, one must operate the plates at thickness resonance, typically Lx=λ/2 resonance. Here Lx is the plate thickness, λ=cl/f is the wavelength of longitudinal waves normal to the plate with wave velocity cl and frequency f. The resonance makes the transducer efficient in a band of frequencies around a center frequency f0=cl/λ0=cl/2Lx. Acoustic matching plates between the ceramic plate and the load are used to improve the power coupling to the load, a technique that increases the bandwidth of the transducer resonance.
With the well known composite technique, where the ceramic plate is diced into small posts, and the interpost space is filled with polymer, the efficient characteristic impedance is reduced to ˜15 MRayl, which is still around 10 times higher than the characteristic impedance of the load, such as soft tissue or water. Transducers of composite material must therefore also operate in thickness resonant mode, albeit one can obtain some wider bandwidth than with the transducers of whole ceramic.
Hence, both with whole and composite ceramic, the resonant operation requires that the plate thickness is inversely proportional to the center frequency of the operating transducer band. This requires thicknesses in the range of 200-20 μm for center frequencies in the range of 10-100 MHz. Today, lapping of the ceramic plate is the common technology to manufacture plates with correct thickness, which becomes difficult and expensive at thicknesses in ranges below 50-60 μm, corresponding to frequencies above 30-40 MHz. Composite ceramic/epoxy material is also difficult to make for frequencies above 15 MHz, and it is hence a general need for efficient methods to manufacture transducers with a functioning high frequency band above ˜10 MHz.
The invention devices a solution to these problems as described below.